Facultative

[fækәltәtiv]

解释:

(adj.) not compulsory; 'facultative courses in the sciences' .

(adj.) granting a privilege or permission or power to do or not do something; 'a facultative enactment' .

(adj.) able to exist under more than one set of conditions; 'a facultative parasite can exist as a parasite or a saprophyte' .

(adj.) of or relating to the mental faculties .

芭比整理--From WordNet